On comparing a CB with a CS I noticed the CB has five screws in a pentagon layout. The CS has six screws in a symmetrical pattern. Is there a rhyme or reason behind the choice F.P. Journe makes in where and how many screws to use on a case back? I have noticed even more interesting layouts on other models.

including material of case, thickness of case, diameter of case, water resistance level, layout of crowns, symmetry desired, and maybe a couple of others that are only known to the designers. The Centigraphe has six screws but they aren't symmetrical due to the crown and pusher for chrono function.